Compressibility is a measure of the relative volume change of a fluid or solid as a response to a pressure (or mean stress) change.
Applicable units are those of quantitykind:Compressibility
http://en.wikipedia.org/wiki/Compressibility
https://cdd.iec.ch/cdd/iec61987/iec61987.nsf/ListsOfUnitsAllVersions/0112-2---62720%23UAD024
Compressibility is a measure of the relative volume change of a fluid or solid as a response to a pressure (or mean stress) change.
$\chi = -(\frac{1}{V})\frac{dV}{d\rho}$, where $V$ is volume and $p$ is pressure.
http://www.iso.org/iso/catalogue_detail?csnumber=31889